Wireless phones and other user equipment supporting a fourth-generation (4G) wireless technology such as the Long Term Evolution (LTE) technology, formally known as Evolved Universal Terrestrial Radio Access (E-UTRA), typically also support a 3G technology, such as the Universal Terrestrial Radio Access (UTRA) technology often referred to as Wideband Code-Division Multiple Access (WCDMA). These same devices might also be compatible with 2G networks, such as the Global System for Mobile Communications (GSM)/EDGE Radio Access Network (GERAN).
These networks may be connected to one another and, in some circumstances, may permit a user equipment (UE) to be handed over from one to another. As shown in FIG. 1, the LTE and GERAN/UTRAN architectures are combined by means of interfaces between the core network nodes of each respective technology. See “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access,” 3GPP TS 23.401, ver. 13.0.0 (September 2014), available at www.3gpp.org. These core nodes include, for example, the Mobility Management Entity (MME), the Serving GPRS Support Node (SGSN), the Serving Gateway (SGW), and the Home Subscriber Server (HSS), all of whose functions are well known to those generally familiar with the family of network standards developed by members of the 3rd-Generation Partnership Project (3GPP).
One of the ways for the LTE and GERAN/UTRAN technologies to communicate with each other is via the RAN Information Management (RIM) protocol, which allows transferring of information from LTE to GERAN/UTRAN and vice-versa in a pre-configured manner. The RIM protocol is specified in “3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; General Packet Radio Service (GPRS); Base Station System (BSS)—Serving GPRS Support Node (SGSN); BSS GPRS Protocol (BSSGP) (Release 12),” 3GPP TS 48.018, v. 12.3.0 (September 2014), also available at www.3gpp.org.
In the current specifications, a specific type of RIM interaction is defined for the purpose of avoiding unnecessary handovers from LTE to GERAN/UTRAN networks. This interaction is known as “Unnecessary IRAT Handover detection.”
FIG. 2 shows a message sequence chart for the Unnecessary Inter Radio Access Technology (IRAT) Handover detection procedure, as per the current standards, where the target RAT is GERAN. A similar message sequence is also valid when the target RAT is UTRAN.
The operations illustrated in FIG. 2 are described in detail in 3GPP TS 48.018 (cited above), 3GPP TS 36.413 (Release 12), 3GPP TS 25.413 (Release 12), and 3GPP TS 48.008 (Release 12), all of which can be found at www.3gpp.org. The illustrated procedure allows the LTE radio access network (RAN) to configure specific measurement criteria and thresholds for a UE that is handed over from LTE to GERAN/UTRAN. More generally, similar procedures may be used when a UE is handed over from a first RAN 204, operating according to a first RAT, to a second RAN 202, operating according to a second RAT.
Referring again to FIG. 2, the measurement configuration sent from the source RAN/RAT (an LTE network, in this case) to the target RAN/RAT (a GERAN/UTRAN network), is captured in the information elements that make up the IRAT Measurement Configuration IE. Details of the IRAT Measurement Configuration IE can be found in 3GPP TS 48.018, 3GPP TS 25.413, and 3GPP TS 48.008; some of those details are illustrated below, in FIGS. 3 and 4. FIG. 3 shows an IRAT Measurement Configuration Information Element (IE). FIG. 4 a structure of an IRAT Measurement Configuration IE as used for handover from LTE to UTRAN. The configuration of FIG. 4 may be sent by LTE to UTRAN/GERAN via the Source BSS to Target BSS Transparent Container IE or Old BSS to New BSS information IE (in case of handover to GERAN; see 3GPP TS 48.018 and 3GPP TS 48.008) or via the Source RNC to Target RNC Transparent Container IE (in case of handover to UTRAN, see 3GPP TS 25.413) within the handover signalling, i.e. as part of the HANDOVER REQUIRED and HANDOVER REQUEST messages generated in LTE (as shown in step 1 and step 2 of FIG. 2).
Upon reception of such configuration, the UTRAN/GERAN will need to configure the UE (handed over from LTE) to perform measurements for a time duration equal to the Measurement Duration IE and over the E-UTRAN frequencies indicated in the E-UTRA Absolute Radio-Frequency Channel Number (E-ARFCN) IE. The LTE cells for which measurements are taken will be recorded by the target UTRAN/GERAN base station if the measurement results are above preconfigured thresholds specified in the REPORTING_THRESHOLD IE or Reference Signal Received Power (RSRP) IE or Reference Signal Receive Quality (RSRQ) IE.
The measurements performed by the UE as a result of the Unnecessary IRAT Handover procedure will trigger the delivery of an HO Report IE from GERAN/UTRAN to E-UTRAN as part of a RIM message (see step 5 of FIG. 2) if the following is satisfied (excerpt from 3GPP TS 25.413 showing the conditions in UTRAN):                HO Report should be sent if there is either a single source RAT cell whose measurement results exceed the threshold for the whole measurement duration, or a group of source RAT cells together provide such coverage. The cells that exceed the threshold in the first UE measurement report are included in the HO Report. If both thresholds are present, the received radio measurements must exceed both the RSRP and the RSRQ thresholds in order to satisfy the indicated radio conditions.        When the HO Report is sent from RNC at the end of the configured measurement duration, it shall set the HO Report Type IE to “Unnecessary HO to another RAT”. If the measurement period expires due to an inter-RAT handover towards LTE executed within the configured measurement duration, the RNC shall set the HO Report Type IE in the HO Report to “Early IRAT Handover”.        No HO Report shall be sent in case no E-UTRAN cell could be included, or if the indicated period of time is interrupted by an inter-RAT handover to a RAT different than LTE or by an intra-UMTS handover with SRNC relocation.        
As can be seen from the quote above, the HO Report IE will be generated only if there are detected cells that satisfy the measurement configuration criteria. Cells can be included in the HO Report only if reported UE measurements for each of the cells detected satisfy the configured thresholds for the whole duration of the configured measurement window or for part of such duration, in the event that the measurement window time is interrupted by an inter-RAT handover towards LTE.
In case all the conditions are satisfied, the HO Report IE sent from UTRAN/GERAN to LTE via RIM is constructed as shown in FIG. 5 (see 3GPP TS 36.413). In the HO Report IE, the cells reported in the Candidate Cell List IE are those LTE cells providing good enough coverage, namely fulfilling the criteria specified in the IRAT Measurement Configuration IE (see FIGS. 3 and 4). Such cells are represented by a list of E-UTRAN Cell Global Identities (E-CGIs). The latter can be seen from the specifications of the Candidate Cell ID IE, which is detailed as shown in FIGS. 6 and 7 (see 3GPP TS 36.413).
The discussion presented herein generally assumes a management system having an arrangement like that shown in FIG. 8. In this arrangement, node elements (NE), also referred to as eNodeBs, are managed by a domain manager (DM), also referred to as the operation and support system (OSS). A DM may further be managed by a network manager (NM). The system composed by the DM and NM may be referred as the Operation and Maintenance System (OAM). Two NEs are interfaced by X2, whereas the interface between two DMs is referred to as Itf-P2P. It is further assumed, in the discussion that follows, that a function that automatically optimizes NE parameters can, in principle, execute in the NE, DM, or NM.